The present invention relates generally to semiconductor dies, and more particularly, to a semiconductor die that has protection on its backside against invasive attacks.
A semiconductor die includes electronic circuits formed on a semiconductor substrate. The die also includes power, ground and signal nets that are connected to the circuits to power the circuits and allow the circuits to communicate with each other. These nets are implemented with conductive wires, which usually are metal traces patterned in various layers of the die.
Semiconductor dies may be used in credit cards, debit cards, smart cards, and so on, such that the wires carry confidential data such as an encryption key, a password and the like. Such dies may be subject to tampering in order to illicitly access the confidential data. Such tampering can be in the form of an invasive attack, a semi-invasive attack or a non-invasive attack.
In an invasive attack, physical properties of the semiconductor die are irreversibly modified to retrieve the confidential data. Invasive attacks are commonly referred to as physical attacks. Micro-probing, focused ion beam (FIB), and decapsulating the semiconductor die are a few examples of physical attacks. In a semi-invasive attack, the confidential data is retrieved without making an electrical contact with the semiconductor die. Ultra-violet attacks, optical fault detection and advanced imaging techniques are a few examples of the semi-invasive attacks. In a non-invasive attack, the confidential data is retrieved by altering the operation of the circuitry, typically by manipulating the supply voltage, the clock signal, and the like. Side-channel attacks, brute channel attacks and fault injection attacks are a few examples of the non-invasive attacks.
In a FIB type invasive attack, a trench is formed, using a beam of ions, on the backside of the semiconductor die to access the conductive wires, and layers of metal ions are deposited in the trench. A microprobe is used to establish electrical connections with the wires via the layers of metal ions deposited in the trench. Then confidential data transmitted across the wires may be retrieved using the microprobe.
A known technique to overcome semi-invasive attacks is to insulate the semiconductor die with a shielding layer. In the absence of the shielding layer, when the backside of the semiconductor die is subjected to a semi-invasive attack and exposed to ultra-violet light, an imprint of the circuitry is revealed, which shows the interconnections among the various circuits. In the presence of the shielding layer, when the backside of the semiconductor die is exposed to the ultraviolet light, the shielding layer blocks the ultraviolet light from accessing the circuitry. However, the deposition of an additional shielding layer increases the overall cost of manufacturing the semiconductor die. Further, the shielding layer does not protect the semiconductor die against physical attacks.
Another known technique is to use dummy lines in addition to the conductive wires. Unlike the conductive wires, the dummy lines do not carry critical information. It is difficult for the attacker to distinguish between the dummy lines and the conductive wires. Thus, the dummy lines successfully protect the semiconductor die against optical attacks. However, the dummy lines do not provide any protection against the aforementioned physical attacks.
Therefore, it would be advantageous to have a protection mechanism that does not increase the cost of manufacturing the semiconductor die and that provides protection against physical attacks.